God Mode:
Press the ~ key to open the console and type "god" without the quotes. This will enable god mode, making you invincible.
Infinite Ammo:
Press the ~ key to open the console and type "ammo" without the quotes. This will give you infinite ammo for all of your weapons.
All Weapons:
Press the ~ key to open the console and type "give all" without the quotes. This will give you all of the weapons in the game.
Unlock All Levels:
Press the ~ key to open the console and type "unlock all" without the quotes. This will unlock all of the levels in the game.
No Clip:
Press the ~ key to open the console and type "ghost" without the quotes. This will enable no clip, allowing you to fly through walls.
Super Jump:
Press the ~ key to open the console and type "superjump" without the quotes. This will give you the ability to jump much higher than normal.
Disable AI:
Press the ~ key to open the console and type "ai disable" without the quotes. This will disable the AI, making the enemies unable to attack you.